Channel-Occupancy Efficient, Low Power Wireless Networking

ABSTRACT

An apparatus and method are provided for efficiently sharing a single wireless channel and for providing improved power saving. Automatic beacon “sliding” establishes a round-robin contention-free channel schedule among multiple IBSSs. Entering an idle state immediately following communication after a beacon saves power. Further power savings occur when presumptions of pending traffic may be made.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.11/775,835, entitled “Channel-Occupancy Efficient, Low Power WirelessNetworking” filed Jul. 10, 2007 which claims priority of U.S.Provisional Patent Application 60/807,042, entitled “Channel-OccupancyEfficient, Low-Power Wireless Networking” filed Jul. 11, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to efficient channel occupancy and powersaving techniques in wireless networks. More particularly, the inventionrelates to media access control (MAC) behavior in wireless networks thatpromotes channel sharing and a greater aggregate idle (as opposed towake or transmit) time among stations.

2. Related Art

The IEEE 802.11-1999 wireless networking standard provides for operationof a plurality of ad-hoc networks (each network forming an independentbasic service set or IBSS) on a single channel. Each IBSS will sendhigh-priority beacon signals in contention with other IBSS beaconsignals. Information traffic, which is of lesser priority than beaconsignals, then contends for the channel when beacon signals are notpresent. As traffic becomes denser, more collisions typically occur,thereby requiring contention resolution in the form of random backoff.

The IEEE 802.11-1999 wireless networking standard (§11.2.2 of thatstandard) provides for an ad-hoc power-saving mode. This power-savingmode calls for each station in the ad-hoc network to listen at beacontime for an indication that traffic for that station is pending. Theseindications are provided in the form of announcement traffic indicationmessage (ATIM) packets. Stations with pending traffic must remain in the“wake” state until the next target beacon transmission time (TBTT)occurs without an ATIM packet for that station. This wake stateundesirably uses more power than an “idle” state.

Wireless networking is being adapted to new applications, such ashandheld interactive video games and voice over Internet Protocol(VoIP), both of which have well-characterized and regularly occurringdata transmissions. For example, a video game may want to update itsstatus relative to other players at approximately a 60 Hz rate. Anexemplary update rate for VoIP could be approximately 33-50 packets persecond. Jitter, which results from packet collision, contention, andresolution, may interfere with smooth game play or voice communication.Therefore, a need arises for channel access that will promote contentionavoidance.

Moreover, power savings are desirable for many wireless applications.For example, handheld interactive video games are commonly powered bybatteries. Therefore, a further need arises for improving power savingin wireless ad-hoc networks carrying relatively dense andwell-characterized traffic.

SUMMARY OF THE INVENTION

Methods for providing access to a wireless channel advantageouslypromote both contention avoidance and power savings. In accordance withone aspect of the invention, a station can enter an idle (i.e. lowpower) state after the station has completed its data packettransactions and reception, but earlier than waiting for the next TBTTand ATIM window. Specifically, using a MORE DATA bit in the framecontrol field in a non-802.11 way can indicate traffic completion.

In one embodiment, a packet with its MORE DATA bit set to “1” indicatesthat the receiving station should remain in a wake state because atleast one more packet for that station is pending. In contrast, a packetwith its MORE DATA bit set to “0” (referenced herein as a NULL packet)indicates that the station can enter an idle state until the next beacontime, assuming that the station is not receiving unicast packets ortransmitting any packets. In one embodiment, a small delay (relative tothe beacon interval) occurs before transmission of the NULL packet toallow for queuing delays from a higher-level networking layer to alower-level transmission queue. This delay can be set by a NULL timer inthe station. If the NULL timer has not expired, then the station cancontinue to receive and/or transmit packets.

In accordance with another aspect of the invention, a self-selected“master” station of an IBSS can enter the idle state after the masterhas received a reply from all other stations (“clients”) in the IBSS. Inthis case, the ATIM window can be set to zero and communication betweenall clients and the master is assumed. This configuration isadvantageous in applications such as game control traffic where eachbeacon interval is known to involve status transfer from the master toall stations and vice versa. In this way, no traffic is wasted sendingnotifications or waiting for the ATIM window to expire.

In accordance with yet another aspect of the invention, multiple masters(each master controlling an IBSS on one channel) may cause theirrespective beacon timings to shift to non-contending times by settingbeacon transmission to a priority below that of data traffic (normally,beacon transmission has a higher priority than data traffic). In thismanner, each IBSS beacon will be followed by a burst of data traffic,wherein the data traffic forces other IBSS masters to wait until thedata traffic completes before sending their beacons. Multiple IBSSs willtherefore “slide” their beacons and data bursts to a temporallyunoccupied portion of a channel, thereby establishing a round-robinaccess pattern between the IBSSs with minimal contention. In oneembodiment, to provide this sliding, the priority of a beacon packet canbe less than a priority of at least one of DIFS packets and EIFSpackets. In this case, the beacon refresh period for each IBSS can beset to at least a sum of data burst lengths for all IBSSs. The DIFSpackets can include at least one of data packets and management packets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art ad-hoc power saving procedure prescribedby the IEEE-802.11-1999 wireless networking standard.

FIG. 2A illustrates a technique including a non-standard use of a MOREDATA bit. This non-standard use allows a station to remain in a wakestate to receive packets and, optionally, to transmit packets to anotherstation.

FIG. 2B illustrates additional steps in the technique of FIG. 2A inwhich a Null timer can be used in combination with the MORE DATA bit.This Null timer can immediately put the station in a power-saving idlestate when reception, transmission, and a small delay time havecompleted.

FIG. 3 illustrates a power saving technique in which each client stationcan enter an idle state after it receives traffic from and providestraffic to a master station. The master station can enter an idle stateafter it has received traffic from all the client stations.

FIG. 4 illustrates the traffic of multiple groups of stations aftersettling into a contention-free timing pattern.

FIG. 5 illustrates various types of AIFS period, e.g. a distributedinter-frame space (DIFS).

DETAILED DESCRIPTION OF THE FIGURES

Various techniques can be used to reduce power consumption andcontention in wireless networking devices. For example, FIG. 1illustrates an ad-hoc power saving technique 100 prescribed by theIEEE-802.11-1999 wireless networking standard. In technique 100, abeacon 104 is followed by an announcement traffic indication message(ATIM) window 111 (first, second, and third ATIM windows 111 shown),where the period of ATIM window 111 is specified by beacon 104.

Notably, any station having data to transmit sends its ATIM within ATIMwindow 111, although the timing of each ATIM transmission is arbitrary.In FIG. 1, only station 101 has pending data to send to station 102, asindicated within the second ATIM window 111 for those stations.

Specifically, station 101 transmits its ATIM (Transmit ATIM 120) andstation 102 receives that ATIM (Receive ATIM 121) within the second ATIMwindow 111. Station 102 then transmits an acknowledgement (ACK) signalof that received ATIM (Transmit ACK 123) and station 101 receives thatACK signal (Receive ACK signal 122). Note that Transmit ACK 123 andReceive ACK 122 could be transmitted and received, respectively, outsideATIM window 111. At this point, station 101 can transmit a data frame(Transmit Frame 124) and station 102 can receive that data frame(Receive Frame 125). After receipt of the data frame, station 102 cantransmit an ACK signal (Transmit ACK 127) and station 101 can receivethat ACK signal (Receive ACK 126).

Notably, stations 101 and 102 stay in a wake state until the end of thenext (i.e. third) ATIM window 111 even though they have finishedtransmitting and receiving packets. In contrast, because station 103 isnot receiving or transmitting packets within the second ATIM window 111,station 103 enters an idle (i.e. low power) state as soon as the secondATIM window 111 concludes. Therefore, stations (e.g. stations 101 and102) that complete their communication well before the end of a beaconinterval 110 continue to use significant power in the wake statecompared to stations in the idle state (e.g. station 103).

FIGS. 2A and 2B illustrate a technique 200 that provides enhanced powersaving for stations in an ad-hoc wireless network. To provide this powersaving, technique 200 uses a MORE DATA bit in a non-802.11 way. The MOREDATA bit, which is provided in a MAC frame format, is typically used todesignate segmentation. Specifically, the MORE DATA bit would indicatewhether a single, extremely large frame from an access point (AP) hasbeen broken up into multiple, smaller frames that are being buffered bythat AP. In this conventional use, a value of “1” indicates that atleast one additional buffered frame will be sent from the AP to thereceiving station.

In accordance with one aspect of the invention, the MORE DATA bit is setto either “1” (which indicates that the receiving station should remainin a wake state because at least one more packet for that station ispending) or “0” (which indicates that this packet includes no data andno further data packets will be sent, thereby providing a potentialadvantage by allowing the station to enter an idle state before a nextbeacon). As explained below, this MORE DATA bit can be advantageouslyused in transmitting broadcast, multicast, and/or unicast packets.

In step 201, a station wakes to receive one or more beacons. Step 202determines whether that station has pending traffic for one or morestations in that network. If so, then step 203 transmits an ATIM tothose stations. An ATIM can be a broadcast ATIM, a multicast ATIM, or aunicast ATIM, wherein each ATIM designates its intended receivingstations.

Following transmission of the ATIM or assuming that the station has nopending traffic for another station, step 204 determines whether anybroadcast (BC) ATIM or multicast (MC) ATIM designates that station. Ifso, then step 205 receives the corresponding BC/MC packet. At thispoint, step 206 determines whether the MORE DATA bit of the BC/MC packetis set to “1”, thereby indicating that at least one more packet for thatstation is pending. If so, then technique 200 returns to step 205 toreceive another BC/MC packet.

If no BC/MC packet is being transmitted (step 204) or if the MORE bit isnot set to 1 (step 206) (i.e. the MORE bit is set to 0, therebyindicating that that this packet includes no data and no further BC/MCpackets will be sent), then step 207 determines whether a unicast ATIMdesignating that station was sent. If so, then step 208 receives thatunicast packet. Note that, in this embodiment, broadcast and multicastpackets are given higher priority than unicast packets because broadcastand multicast packets affect more stations and typically involveimportant functions.

After receiving a unicast packet, step 209 determines whether the MOREDATA bit is set to 1, thereby indicating that at least one more unicastpacket for that station is pending. If so, then step 210 sets a NULLtimer in the station and the station returns to step 208 to receiveanother unicast packet. Note that if multiple unicast packets for thatstation are pending, then each traversal of step 210 resets the NULLtimer.

If no unicast packet is pending for that station (step 207) or if theMORE DATA bit is not set to 1 (step 209) (i.e. is set to 0), then step211 determines whether that station has any pending traffic for anotherstation (or a set of stations using broadcast or multicast packets). Ifso, then step 213 sets the NULL timer. Step 214 determines whether atleast one more packet for the destination station(s) is pending. If so,then step 215 transmits a packet with its MORE DATA bit set to 1. Then,technique 200 returns to step 213 to reset the NULL timer.

If another packet for another station is not pending (step 214), thenstep 218 determines whether a timeout of the NULL timer has occurred. Ifso, then step 217 determines whether the station sent any data. If so,then the station sends a NULL packet, i.e. a packet with its MORE bitset to “0”. This NULL packet indicates to the current receivingstation(s) that the current packet includes no data and no furtherpackets with data from the transmitting station will be sent in thisbeacon interval. Therefore, the NULL packet in combination with the MOREDATA bit set to 0 can advantageously indicate that other stations canenter an idle state (assuming the other conditions discussed above aremet). If no traffic for other station(s) is pending (step 211) or aftersending the null packet (step 216), then step 212 enters an idle state.Note that if the null timer has not timed out (step 218), then technique200 returns to step 207 to determine whether any other unicast packetfor that station is pending.

Technique 200 advantageously provides power savings based on thefollowing flow summary.

(1) A station will enter the wake state to receive a beacon signal.

(2) Following that beacon, a station will send any ATIM transmissions.

(3) A station, upon receiving an ATIM indicating broadcast or multicasttraffic, will receive such traffic until a packet with the MORE bit isset to 0 (clear or cleared).

If no unicast ATIM indicating packets queued for this station has beenreceived and no ATIM was transmitted from this station, then the stationcan immediately enter an idle state.

(4) If a unicast ATIM for this station was received or if an ATIM wastransmitted from this station, then the station can receive and/ortransmit data (in one embodiment, receiving packets (if any) first andthen transmitting packets (if any)). As long as packets are beingreceived or transmitted with the MORE DATA bit being set to 1, a NULLpacket timer is being set to a predetermined delay value. After a NULLpacket has been received or transmitted by the station, therebyindicating that no additional queued packets are pending (for eitherreceipt or transmission), the station returns to the power save idlestate.

In one embodiment, the predetermined delay value is approximately 10 ms.This delay advantageously keeps the station in the wake state even if ahigher data transmission protocol layer cannot fill the packettransmission queue quickly enough to prevent the queue from appearingempty and causing the transmission of the NULL packet with the MORE DATAbit cleared prematurely. This delay value can be tuned for particularapplications.

In some embodiments, a station receiving packets may immediately sendpackets back to the originating station without intervening signals.Note that this immediate transmission is in contrast with the practiceof the IEEE-802.11-1999 wireless networking standard, which wouldnormally require a response to be sent after another beacon and thetransmission of an ATIM by the responding recipient. This immediatetransmission and the predetermined delay provided by the NULL timerpermit throughput substantially similar to that of the IEEE-802.11-1999wireless networking standard without requiring a station to remain inthe wake state for the duration of the target beacon transmission time(TBTT).

FIG. 3 illustrates an IBSS in which one station is self-designated amaster station (“master”) 301 and multiple client stations (“clients”)want to receive data from and send data to the master in each beaconperiod. When master 301 wakes to send a beacon 310, clients 302, 303,and 304 enter the wake state to hear the beacon. At this point, master301 can send data, i.e. keyset packets 311, to clients 302, 303, and 304(shown by down arrows). A keyset packet 311 includes aggregated, updateddata from all clients. To ensure that all clients receive the keysetpacket 311, master 301 sends a unicast packet to each client (note thattransmitting a multicast packet would not necessitate a correspondingACK packet that acknowledges receipt of the keyset packet).

Once clients receive their keyset packets 311, each client can respondto master 301 with its corresponding key packet (i.e. updated data fromthat station) (shown by up arrows) based on wireless mediumavailability. That is, each client contends with the other clients aswell as master 301 for the wireless medium. For example, after receivingits keyset packet 311, client 302 must wait for an open period on thewireless medium to send its corresponding key packet 312 to master 301.In this case, the first opportunity for this transmission is aftermaster 301 sends a keyset packet 311 to client 303. Note that becauseclient 303 has also received its keyset packet 311 by this time, clients302 and 303 would both be vying to send their key packets back to master301. In this case, client 302 “beat” client 303 in sending its keypacket back to master 301. Therefore, client 303 waits until the nextopen period on the wireless medium to send its key packet 313 to master301. Once again, because both clients 303 and 304 have received theirkeyset packets 311 by that time, clients 303 and 304 would be vying tosend their key packets back to master 301. In this case, client 303“beat” client 304 in sending its key packet back to master 301.

As shown in FIG. 3, after concluding reception and transmission, eachclient can immediately enter an idle state until it wakes to listen forthe next beacon (not shown). After clients 301, 302, and 303 haveconcluded their receptions and transmissions, master 301 can enter theidle state.

Notably, FIG. 3 illustrates an ATIM-less process in which each client ispresumed to want to receive data from and send data to the master ineach beacon period. This presumption advantageously saves transmit powerthat would otherwise be expended communicating the obvious, i.e. packetsare to be transmitted by each station (both master and clients) eachbeacon period. Note that the transmit state consumes significantly morepower than the wake state. Therefore, an ATIM-less process can savesignificant power.

In one embodiment, the stations can still send ATIMs, wherein eachclient can enter the idle state after that client concludes itstransactions with the master. In this case, even without an ATIMpresumption, each client can still enter the idle state earlier thanprescribed in the IEEE-802.11-1999 wireless networking standard, therebystill saving power.

FIG. 4 depicts IBSSs 401, 402, and 403 after having reached asteady-state round-robin distribution of their transmission times on ashared channel. This steady state can be established by setting thepriority of beacons below that of other packets (note that beaconstypically have highest priority). As explained below, beacons from otherIBSSs will be forced to wait for the completion of a data burstassociated with one IBSS before the next beacon can be sent.

In one embodiment, a beacon can be set to use an arbitration inter-framespace (AIFS) (i.e. a predetermined space between frames) of 5 with acontention window (CWIN) size of 7 relative to data with an AIFS of 1and a CWIN size of 3. The IEEE-802.11-1999 standard defines, in §9.2.3,several types of AIFS periods, including a distributed interframe space(DIFS) (shown in FIG. 5 for reference purposes) and an extendedinterframe space (EIFS). The DIFS (DCF interframe space) is used bystations operating under the DCF to transmit data frames and managementframes. The EIFS (extended interframe space) is used by the DCF wheneverthe PHY has indicated to the MAC that a frame transmission that starteddid not result in a correct reception.

In one embodiment, beacons from each IBSS will be forced to “slide” intoa distinct position relative to the other IBSSs, thereby establishing asteady-state round-robin temporal distribution that minimizes collisionsbetween the IBSSs. For example, a station in IBSS 401 can transmit itsbeacon 411 during beacon interval 404A (note that only one station in anIBSS generally transmits beacons), a station in IBSS 402 can transmitits beacon 412 during beacon interval 404B, and a station in IBSS 402can transmit its beacon 413 during beacon interval 404C.

After IBSSs 401, 402, and 403 have used their respective beaconintervals 404A, 404B, and 404C for transmission (e.g. for beacons,ATIMs, data frames, ACKs, etc.), the beacon-generating station in IBSS401 can once again transmit its beacon during beacon interval 404D(partially shown). IBSSs 402 and 403 can follow the established prioritypattern. This round-robin arrangement can be established by setting anappropriate beacon refresh period for each IBSS that is equal to atleast the aggregate of the beacon intervals for all IBSSs. For example,if each beacon interval 404A, 404B, and 404C is 5 msec, then the beaconrefresh period for each IBSS is at least 15 msec. Notably, thisarrangement works well so long as the data burst length is less thanTBTT/N, wherein there are N IBSSs active on one channel.

In another embodiment, the priority of a beacon packet can be less thana priority of at least one of DIFS packets and EIFS packets. In thiscase, the beacon refresh period for each IBSS can be set to at least asum of data burst lengths for all IBSSs. As noted above, the DIFSpackets can include at least one of data packets and management packets.

Although illustrative embodiments of the invention have been describedin detail herein with reference to the accompanying figures, it is to beunderstood that the invention is not limited to those preciseembodiments. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed. As such, many modificationsand variations will be apparent. For example, although the use ofbeacons is described herein, other types of coordination transmissions,e.g. a contention-free poll (CF-Poll) packet can be used in the presentinvention. Accordingly, it is intended that the scope of the inventionbe defined by the following Claims and their equivalents.

1. A method of saving power in a station, the station forming part of anad-hoc wireless network including a plurality of stations, the methodcomprising: waking to receive a beacon; transmitting any ATIM; if abroadcast/multicast (BC/MC) ATIM indicating pending BC/MC packets isreceived, then receiving the BC/MC packets until a packet with its MOREDATA bit is set to a first predetermined value; if a unicast ATIMindicating pending unicast packets is received, then receiving theunicast packets until a packet with its MORE DATA bit is set to thefirst predetermined value; if an ATIM was transmitted and at least onepacket for another station is pending, then transmitting the packet withits MORE DATA bit set to a second predetermined value; if no unicastATIM is received and no ATIM was transmitted, then entering an idlestate before a next beacon.
 2. The method of claim 1, further including:setting a NULL timer to a predetermined delay value as long as packetsare being received or transmitted with the MORE DATA bit being set tothe second predetermined value.
 3. The method of claim 2, furtherincluding: after a NULL timer timeout, sending a NULL packet with itsMORE DATA bit set to the first predetermined value if the station sentany data and then entering the idle state.
 4. A method of saving powerin an independent basic service set (IBSS), the IBSS including a masterand a plurality of clients, the method including: presuming that themaster communicates with the plurality of clients and each of theplurality of clients communicates with the master during each beaconinterval; sending a keyset packet from the master to each of theplurality of clients during a beacon interval, the keyset packetincluding an aggregate of all current client data; and sending a keypacket from each client to the master during the beacon interval, thekey packet including new client data, wherein each client enters an idlestate after sending its key packet, wherein the master enters the idlestate after sending its keyset packet to the plurality of clients, andwherein the master and the plurality of clients contend for a sharedwireless medium.
 5. A method of saving power in an independent basicservice set (IBSS), the IBSS including a master and a plurality ofclients, the method including: presuming no announcement trafficindication message (ATIM) communication between the master and theplurality of clients; sending a keyset packet from the master to each ofthe plurality of clients during a beacon interval, the keyset packetincluding an aggregate of all current client data; and sending a keypacket from each client to the master during the beacon interval, thekey packet including new client data, wherein each client enters an idlestate after sending its key packet, wherein the master enters the idlestate after sending its keyset packet to the plurality of clients, andwherein the master and the plurality of clients contend for a sharedwireless medium.
 6. A method of minimizing contention between multipleindependent basic service sets (IBSSs), the method comprising: setting apriority of a beacon packet less than a priority of any other type ofpacket; and setting a beacon refresh period for each IBSS equal to atleast a sum of beacon intervals for all IBSSs.
 7. A method of minimizingcontention between multiple independent basic service sets (IBSSs), themethod comprising: setting a priority of a beacon packet less than apriority of at least one of DCF interframe space (DIFS) packets andextended interframe space (EIFS) packets; and setting a beacon refreshperiod for each IBSS equal to at least a sum of data burst lengths forall IBSSs.
 8. The method of claim 7, wherein DIFS packets are at leastone of data packets and management packets.
 9. A method of saving powerin a station, the station forming part of an ad-hoc wireless networkincluding a plurality of stations, the method comprising: waking toreceive a coordination transmission; transmitting any ATIM; if abroadcast/multicast (BC/MC) ATIM indicating pending BC/MC packets isreceived, then receiving the BC/MC packets until a packet with its MOREDATA bit is set to a first predetermined value; if a unicast ATIMindicating pending unicast packets is received, then receiving theunicast packets until a packet with its MORE DATA bit is set to thefirst predetermined value; if an ATIM was transmitted and at least onepacket for another station is pending, then transmitting the packet withits MORE DATA bit set to a second predetermined value; if no unicastATIM is received and no ATIM was transmitted, then entering an idlestate before a next coordination transmission.
 10. The method of claim9, further including: setting a NULL timer to a predetermined delayvalue as long as packets are being received or transmitted with the MOREDATA bit being set to the second predetermined value.
 11. The method ofclaim 10, further including: after a NULL timer timeout, sending a NULLpacket with its MORE DATA bit set to the first predetermined value ifthe station sent any data and then entering the idle state.
 12. A methodof saving power in an independent basic service set (IBSS), the IBSSincluding a master and a plurality of clients, the method including:presuming that the master communicates with the plurality of clients andeach of the plurality of clients communicates with the master duringeach coordination transmission interval; sending a keyset packet fromthe master to each of the plurality of clients during a coordinationtransmission interval, the keyset packet including an aggregate of allcurrent client data; and sending a key packet from each client to themaster during the coordination transmission interval, the key packetincluding new client data, wherein each client enters an idle stateafter sending its key packet, wherein the master enters the idle stateafter sending its keyset packet to the plurality of clients, and whereinthe master and the plurality of clients contend for a shared wirelessmedium.
 13. A method of saving power in an independent basic service set(IBSS), the IBSS including a master and a plurality of clients, themethod including: presuming no announcement traffic indication message(ATIM) communication between the master and the plurality of clients;sending a keyset packet from the master to each of the plurality ofclients during a coordination transmission interval, the keyset packetincluding an aggregate of all current client data; and sending a keypacket from each client to the master during the coordinationtransmission interval, the key packet including new client data, whereineach client enters an idle state after sending its key packet, whereinthe master enters the idle state after sending its keyset packet to theplurality of clients, and wherein the master and the plurality ofclients contend for a shared wireless medium.
 14. A method of minimizingcontention between multiple independent basic service sets (IBSSs), themethod comprising: setting a priority of a coordination transmissionpacket less than a priority of a data transmission packet; and setting acoordination transmission refresh period for each IBSS equal to at leasta sum of coordination transmission intervals for all IBSSs.
 15. A methodof minimizing contention between multiple independent basic service sets(IBSSs), the method comprising: setting a priority of a coordinationtransmission packet less than a priority of at least one of DCFinterframe space (DIFS) packets and extended interframe space (EIFS)packets; and setting a coordination transmission refresh period for eachIBSS equal to at least a sum of data burst lengths for all IBSSs. 16.The method of claim 15, wherein DIFS packets are at least one of datapackets and management packets.